1. Field of the Invention
The present invention relates to a color cathode ray tube, and more specifically to an improvement of an electron gun assembly thereof.
2. Description of the Related Art
Conventionally, an electron gun assembly of an inline type is used in a color cathode ray tube. It includes three electron guns arranged in line with one another. The resolution characteristic of the color cathode ray tube with this arrangement is lowered by a deflective aberration such that beam spots on a phosphor screen become greater as electron beams are deflected from the center region of the screen toward the peripheral region thereof. Supposedly, this aberration consists of two superposed deflective aberrations.
A first deflective aberration is caused since the more the electron beams are deflected, the longer the paths of the electron beams from the electron guns to the phosphor screen become. If a proper focusing voltage is applied to the electron guns, the focused electron beams can form small enough beam spots in the center region of the phosphor screen. In the peripheral region of the screen, however, the electron beams are over-focused, so that the beam spots are subject to the deflective aberration.
A second deflective aberration is caused due to the nonuniformity of deflection magnetic fields. Thus, in the color cathode ray tube having the in-line electron gun assembly therein, a pincushion-shaped horizontal deflection magnetic field and a barrel-shaped vertical deflection magnetic field are formed as shown in FIGS. 1A and 1B, respectively. Electron beams 21, 22 and 23 impinge on a same position of the phosphor screen by these magnetic fields. In these nonuniform magnetic fields, beams 21, 22 and 23 are subjected to a diverging effect in the horizontal direction and a converging effect in the vertical direction. Thus, the beams are distorted or extended horizontally. Such a deformation, i.e., deflective aberration, is particularly great in the peripheral region of the phosphor screen, so that the resulting beam spots are noncircular.
Under the influences of the first and second deflective aberrations, the electron beams are landed on the phosphor screen, tracing in the manner shown in FIG. 2. In FIG. 2, full lines indicate a path within a horizontal plane, while broken lines indicate a path within a vertical plane. If the electron beams, within the horizontal and vertical planes, are deflected from the center region to the peripheral region of phosphor screen 18, with the focusing voltage for the electron guns adjusted so that the beams are focused on the center region, the beams are subjected to the diverging effect, influenced by the second deflective aberration within the horizontal plane, due to the presence of the nonuniform deflection magnetic field formed by deflection yoke 12. Thus, the electron beams are under-focused on phosphor screen 18. The wider the deflection angle of the electron beams, the longer is the beam path under the influence of the first deflective aberration. Accordingly, the electron beams are over-focused on phosphor screen 18. This over-focusing effect, however, is reduced by the under-focusing effect produced under the influence of the second deflective aberration. As indicated by numeral 19 in FIG. 2, therefore, a focusing plane within the horizontal direction is synthetically formed inside phosphor screen 18, that is, on the electron gun electron gun assembly side thereof. If the electron beams, within the horizontal and vertical planes, are deflected from the center region to the peripheral region of the phosphor screen in a manner such that the beams are focused on the center region, the beams are subjected to the converging effect, influenced by the second deflective aberration within the vertical plane, due to the presence of the nonuniform deflection magnetic field. Thus, the electron beams are over-focused on phosphor screen 18. The wider the deflection angle of the electron beams, the longer is the beam path under the influence of the first deflective aberration. Accordingly, the electron beams are additionally over-focused on phosphor screen 18. As indicated by numeral 20 in FIG. 2, therefore, a focusing plane within the vertical direction is synthetically formed inside the horizontal focusing plane 19, that is, on the assembly side thereof.
Due to these influences of the deflective aberrations on the electron beams, as shown in FIG. 3, circular beam spot 24 is formed in the center region of the phosphor screen, while noncircular beam spots, each consisting of high-luminance core 26 and low-luminance halo 27, are formed in the peripheral region of the screen. Thus, the resolution is degraded in the peripheral region.
Conventionally proposed in Japanese Patent Disclosure No. 61-74246 is a method for correcting the distortion of the beam spots in the peripheral region of the phosphor screen. A quadra-potential electron gun assembly disclosed therein comprises cathodes 1 and first, second, third, fourth, fifth, and sixth grids 2, 3, 4, 5, 6 and 7, as shown in FIG. 4. Fourth grid 5 is composed of first, second, and third members 8, 9 and 10. First and third members 8 and 10 each have three electron circular beam apertures, while second member 9 have horizontally elongated, rectangular electron beam apertures 14. Predetermined voltage V.sub.2 is applied to first and third members 8 and 10, and dynamic voltage Vd, which changes depending on the deflection amount or deflection angle of the electron beams, is applied to second member 9. If the deflection amount of the electron beams is zero, dynamic voltage Vd has the same level as predetermined voltage V.sub.2. As the deflection amount increases, the level of voltage Vd lowers gradually from V.sub.2. Thus, asymmetrical lenses are formed between three members 8, 9 and 10, which constitute fourth grid 5, only if the electron beams are deflected.
In the electron gun assembly disclosed in Japanese Patent Publication No. 61-74246, the asymmetrical lenses apply strong and weak focusing effects to the electron beams passing through the lenses, within the vertical and horizontal planes, respectively. Accordingly, the electron beams should be deformed into the shape of an oval having its major axis within the horizontal plane, and should be incident on a main lens between fifth and sixth grids 6 and 7. In order to form the asymmetrical lenses, as seen from the electrode arrangement shown in FIG. 4, dynamic voltage Vd must be lowered as the deflection amount increases. If dynamic voltage Vd is lowered, the focusing power of a unipotential lens between third and fifth grids 4 and 6 is enhanced, so that the electron beams are positively over-focused on the peripheral region of the phosphor screen. Thus, in the electrode arrangement of FIG. 4, the first deflective aberration becomes so great that the focusing effect of the electron beams in the peripheral region of the phosphor screen will be degraded. The oval-sectioned electron beams incident on the main lens are subjected to strong and weak focusing effects within the horizontal and vertical planes, respectively, due to the spherical aberration of the main lens. These focusing effects are exerted on the electron beams so as to cancel the diverging and focusing effects within the horizontal and vertical planes, which are exerted on the electron beams in nonuniform magnetic fields and cause the second deflective aberration. Thus, according to this patent disclosure, the deflective aberrations are said to be reduced, and the resolution is said to be restrained from being lowered in the peripheral region of the phosphor screen.
In their consideration and discussion as described above, however, the fact that the asymmetrical lens themselves will function astigmatically so as to enhance the second deflective aberration has been ignored.
That is, in order to form the horizontally elongated beam shape in the region between the asymmetrical lens and the main lens, the asymmetrical lens must function so as to converge the beams strongly in the vertical direction and so as to diverge or converge the beams weakly in the horizontal direction.
Such astigmatic functions of the asymmetrical lens coincide with those of the second deflective aberration of the deflection yoke. Thus, the second deflective aberration will be also enhanced so that the resolution in the peripheral region of the phosphor screen may be degraded.
In contrast with the arrangement disclosed in Japanese Patent Disclosure No. 61-74246, a proposal can be deduced such that voltage Vd, whose level changes in synchronism with current 28 supplied to deflection yoke 12, as indicated by numeral 29 of FIG. 5B, is applied to member 9 of fourth grid 5. In proposal, as shown in FIGS. 5A and 5B, voltage Vd has the same level as predetermined voltage V2 when the deflection amount is zero. As the deflection amount increases, the level of voltage Vd rises gradually. According to this proposal, the deflective aberrations can be corrected by applying voltage Vd to member 9 of fourth grid 5. In the electron gun assembly in which the voltage as shown in FIG. 5B is applied to member 9 of fourth grid 5, asymmetrical lenses 16 are formed between three members 8, 9 and 10 of fourth grids 5, as shown in FIG. 6, only if the electron beams are deflected. As shown in FIG. 6, moreover, symmetrical lenses 17 are formed individually between third grid 4 and first member 8 of fourth grid 5 and between fifth grid 6 and third member 10 of fourth grid 5. Asymmetric lenses 16 exert a weak converging effect on the electron beams within the horizontal plane, and a diverging effect on the beams within the vertical plane. Thus, after passing through lenses 16, the electron beams are deformed into the shape of an oval having its major axis within the vertical plane. Also in FIG. 6, broken lines indicate an electron beam path within the vertical plane, while full lines indicate a path within the horizontal plane.
In the electron gun assembly having the electrooptical system shown in FIG. 6, the diverging effect within the vertical plane is exerted so that the beam spots are under-focused on phosphor screen 18. Therefore, the beam spots can be prevented from being over-focused within the vertical plane due to the second deflective aberration. Accordingly, focusing plane 20 on which electron beams are focused in the vertical direction can be brought close to phosphor screen 18. Since the weak converging effect within the horizontal plane acts so that the beam spots are slightly overfocused, focusing plane 19 on which electron beams are focused in the horizontal direction is moved from the side of screen 18 toward the electron gun assembly. As a result, focusing planes 19 and 20 within the vertical and horizontal directions can be made coincident in the peripheral region of phosphor screen 18. Thus, the second deflective aberrations are reduced.
However, if the focusing plane within the horizontal and vertical directions are coincident in the peripheral region of phosphor screen 18, then they are formed on the same side of screen 18 as the electron gun assembly. Within the horizontal and vertical planes, therefore, the beam spots are over-focused and cannot have their minimum possible diameter. This is because asymmetrical lenses 16 are so much weaker than symmetrical lenses 17 that the first deflective aberration can be corrected only insufficiently although the second deflective aberration is properly corrected. Thus, the resolution in the peripheral region of the phosphor screen cannot be fully improved. In order to further improve the resolution, this system should be combined with a dynamic focusing system such that the first deflective aberration is positively compensated by raising the voltage of fifth grid 6, as the deflection amount increases, weakening the focusing effect of main lens 15. This dynamic focusing system, however, requires a voltage modulator circuit as well as dynamic voltage Vd. Also, a dynamic focusing circuit requires withstand voltage compensation, since the reference voltage is at least several kilovolts. Thus, the visual display unit may possibly be increased in costs.